It is known that many digestive tract tumor diseases such as an esophagus cancer are formed from a basal layer within epitheliums, which is the most outer layer of a mucous membrane of a digestive tract. As the malignant degree increases, abnormal cells formed from the basal layer increase and, at last, the entire of epitheliums is replaced. The tumor change in the epitheliums involves a cell simple variant and pathological changes in structure, that is so-called a structural variant. As a result, an irregular tissue arrangement is exhibited which is different from a normal pathological image.
An object of an endoscopic diagnosis is to find this kind of tumor as early as possible. Finding this kind of tumor at an earlier stage may increase the possibility for curing the tumor completely increases by performing a less invasive operation such as an endoscopic treatment.
However, some kinds of tumor such as an esophagus cancer do not have clear form (that is, a polyp or subsidence form) very much at an earlier stage and cannot be always found easily.
Many propositions have been made so far for finding and discriminating tumors having poor changes in form at an earlier stage.
Scattering spectroscopy and scattering imaging (as disclosed in Japanese Unexamined Patent Application Publication No. 2002-95635) is regarded as a leading technology among them. The scattering spectroscopy and scattering imaging are a technology for finding an early change which is difficult to find on a general observation image by optically capturing the scattering change based on a fact that nucleus and structural variants may cause an optical scattering change.
Conventionally, many propositions each using a polarizing optical system has been made in order to measure and/or image a scattering characteristic of the inner part of epitheliums. While a rear single light scattered from the surface of the epitheliums holds a polarized component, multiple light scattered from the inner layer of the epitheliums (such as a mucous membrane layer or a mucous membrane inner layer) are not polarized. Based on the knowledge and based on differential observation values of the horizontal and vertical polarized components, the scattering characteristic is imaged in the proposed technologies.
By illuminating a living body tissue with observation light polarized in a certain direction (such as the horizontal direction), the rear scattering light from a cell arrangement of the surface of the epitheliums can be observed as a polarized component in the same direction (such as the horizontal direction). On the other hand, the light propagated to the inner part of the epitheliums is not polarized due to the multiple scattering effect because of a structure on a cell and/or various tissues can be observed as scattering light reflected by the surface of the tissue.
By observing the light by using a polarizer in a different direction (such as the vertical direction) from that of the observed light, the magnitude of multiple scattered light can be estimated. The value is used to correct the influence of the multiple scattering included in the observed light (horizontally polarized light) maintaining most polarized light by performing a differential operation and to extract single scattered light from cells of the surface layer of the epitheliums.
The single scattering phenomenon from cells can be modeled as Mie scattering from various spherical particles floating in protoplasm. A characteristic of Mie scattered light is that the scattering spectrum form depends on a size of a scattering particle, a refractive index ratio with respect to a peripheral medium (mainly protoplasm in this case) and a observation wavelength. Especially, the relationship between the particle size and the spectrum form is important.
The particle size of the epithelium of the mucous membrane can be estimated by fitting the spectrum form of the single scattered light extracted by the measurement of the polarized light by using a Mie scattering model and by using different particle sizes and the non-linear least square technique, for example.
A cell nucleus is considered as one of main elements contributing to the scattering in the epitheliums. Therefore, it is considered that the particle size estimated by the technique has a high correlation with the size of the cell nucleus.
Since the above-described nucleus variant involves a nucleus swelling (which means that the size of the nucleus increases from the normal size with the tumor changes), the estimation of the size of the nucleus by using the technique allows the estimation of a state of the tumor change in the epitheliums.
Therefore, spectroscopy using polarized light and imaging have a possibility to image a nucleus swelling.
As described above, the spectroscopy and imaging by using polarized light may image a nucleus swelling quantitatively. However, the application to an endoscope may cause problems below:                A special scope self-containing a polarizing optical system is required;        A highly sensitive image pickup element (an optical element for generating and receiving polarized light) is required because polarized light is used (the light energy extremely decreases when a polarizer is used); and        A device is required for obtaining angles of illumination light and observation light (rear scattering angles) precisely in order to model based on Mie scattering (in Mie scattering model, the angle of observed rear scattered light largely depends on the spectrum form).        
By overcoming these problems, high-performance scattering imaging apparatus may be achieved. However, the problems still remain from the viewpoint of the cost of the apparatus.
The present invention was made in view of the problems. It is an object of the invention to provide an imaging apparatus which can perform easy scattering imaging only by improving a light source device and the internal part of a processor when an existing endoscopic optical system is used as it is.